The present invention relates to an austenitic stainless steel.
Austenitic stainless steels have been widely used in various fields for their excellent corrosion resistance, but they have a defect in that they have relatively high susceptibility to stress corrosion cracking.
Stress corrosion cracking includes intergranular stress corrosion cracking which is very often seen in applications where the steels are exposed to high-temperature and high-pressure water, such as piping in a nuclear reactor. Stress corrosion cracking also includes transgranular stress corrosion cracking which is very often seen when the steels are exposed to chloride media such as sea-water heat exchangers.
The intergranular stress corrosion cracking is considered to be caused by a chromium impoverished layer due to intergranular precipitation of chromium carbide. B. F. Wilde and J. E. Weber studied the effects of carbon content on the stress corrosion cracking time of a sensitized 18Cr-9Ni stainless steel in a high-temperature and high-pressure water (289.degree. C.) containing 100 ppm oxygen, and reported that with carbon contents of less than 0.02% no stress corrosion cracking is seen as shown in FIG. 1 and a welded 304-L stainless steel can be safely used in the media (Brit. Corr. J. 4, 42, 1969).
Also it is said that transgranular stress corrosion cracking is promoted by impurities such as P, Mo and N. It has been disclosed in U.S. Pat. No. 3,486,885 that the intergranular corrosion resistance of a high-purity austenitic stainless steel with lowered phosphorus and sulfur contents is satisfactory when its carbon content is lowered to 0.02%.
However, effects of impurities on the resistance to the intergranular stress corrosion cracking have not been clarified, and an austenitic stainless steel having excellent resistance both to the intergranular and transgranular stress corrosion crackings has not been developed.
Conventional Type 304 and Type 316 stainless steels and Type 321 stainless steel stabilized by titanium and Type 347 stainless steel stabilized by niobium can not be used safely in high-temperature and high-pressure water and chloride environments.